1. Field of the Invention
The present invention relates generally to a fuel cell system. More particularly to a fuel cell system capable of supplying new catalyst particles to a carbon monoxide purifier, the reaction efficiency of which is reduced.
2. Description of the Related Art
A conventional fuel cell system is an electricity generating system that directly converts the chemical reaction energy, which is generated by the reaction of a fuel gas and an oxidant gas, into the electrical energy. Recently, as the problems concerning the exhaustion of energy sources and the environmental pollution, this fuel cell is used in various fields such as non-polluting motor vehicles, household electricity generating systems, mobile telecommunication equipment, medical instruments, military equipment, and so on.
The conventional fuel cell system includes, for example, a stack, a fuel processing unit and a fuel tank. The fuel processing unit includes a reformer that reforms fuel to generate a hydrogen gas, and a carbon monoxide purifier which reduces a concentration of carbon monoxide contained in the hydrogen gas. One example of the carbon monoxide purifier is a preferential CO oxidation (PROX) reactor which is well-known in the art.
The carbon monoxide purifier includes a reactor main body having an internal space, and an oxidation catalyst bed filled in the reactor main body. Each oxidation catalyst has a structure in which each pellet-like carrier composed of alumina (Al2O3), silica (SiO2), etc. carries at least one catalytic material such as platinum (Pt) or ruthenium (Ru). These oxidation catalysts are stratified to form the oxidation catalyst bed.
Accordingly, when the hydrogen gas generated from the reformer is supplied into the reactor main body, and then passes through the oxidation catalyst bed, the carbon monoxide contained in the hydrogen gas causes a selective oxidation reaction, so that it is changed into carbon dioxide. Accordingly, the concentration of carbon monoxide is reduced.
In this carbon monoxide purifier, with the lapse of time, the oxidation catalyst bed is poisoned to deteriorate the activity of the catalyst, so that the reaction efficiency of the carbon monoxide purifier is reduced. For this reason, in the conventional carbon monoxide purifier, a thickness of the oxidation catalyst bed is made sufficiently thick, so that, although the activity of the catalyst situated upstream is deteriorated, the catalyst situated downstream allows the carbon monoxide to facilitate the oxidation reaction.
However, because the conventional carbon monoxide purifier includes a thick catalyst bed in consideration of the durability of the oxidation catalyst, it cannot help increasing its volume. Consequently, the conventional fuel cell system cannot be downsized.
Further, since the oxidation catalyst bed is formed to have a thickness thicker than required, the initial cost of production is increased, and the loss of pressure is increased when the hydrogen gas passes through the thick oxidation catalyst bed. As a result, the operation energy of the fuel cell system is increased.
In addition, when the conventional carbon monoxide purifier does not provide the oxidation catalyst bed with a sufficient thickness due to the restriction of its internal space, the carbon monoxide purifier, the durability of which comes to an end, must be periodically replaced. Thus, the fuel cell system increases the cost of maintenance.